Fluoride type glasses, normally containing fluorozirconate (ZrF4) and optic fibres formed from such glasses are disclosed in Br Telecom Technol J vol 5 No 2 Apr. 1987 "Progress in Fluoride Fibres for Optical Communications"--P W France et al and in a paper entitled "Properties of Fluorozirconate Fibres for Applications in the 0.5 to 4.5 Micrometre Region" by P W France et al which was presented at the SPIE Conference at San Diego in August 1987. Such fluoride fibres are the subject of European Patent Application No. 85304280.2 (Publication No. 170380). These three documents are incorporated herein by reference and it is to be understood that the term "fluoride glass" includes but is not limited to all the fluoride-containing glasses described in the above papers and all the fluoride-containing glasses within the scope of the disclosure of the above patent application.
Fluoride glass optic fibres have the capability to transmit wavelengths in the far infra red region of the spectrum and have many applications, particularly in the area of optical fibre sensors. However, if such devices are to be used to best advantage then there is a requirement for all of the ancillary equipment that is used in silica fibre systems, including directional couplers, to be composed of fluoride glass optic fibres.
A method of forming an optical coupler component is known which comprises the steps of forming a channel in a block of substrate material, the channel being curved along its length, laying a length of optic fibre in such channel and bonding the optic fibre to a curved interior surface of the channel, and removing substrate material, and at least cladding material of the optic fibre on the convex side of said interior surface to form a face which exposes a length of at least the cladding of the embedded optic fibre, and polishing said face. An optical coupler can the be formed by bringing together two such optical coupler components at their polished faces with the exposed portions of the optic fibres at least partially superimposed.
However we have found that the glass substrates used in the manufacture of optical coupler components for silica fibres are unsuitable for the manufacture of similar optical coupler components utilising optic fibres of fluoride glass.
In order to obtain optical coupler components of reproducible properties, it is necessary to use a substrate material of a hardness similar to that of the fluoride glass of which the optic fibres are composed. Furthermore we have found that the aqueous polishing media conventionally used for polishing away the substrate and optic fibre material of conventional silica fibre optical coupler components result in undesirable chemical attack on fluoride optic fibres when used to manufacture optical coupler components utilising such fibres. this results in unpredictable effects on the coupling properties of the resulting component. Furthermore, we have found that the flexible band saws used to form the channels in the optical components in the prior art leads to random variations in the radius of curvature of the channel which in turn affects the coupling properties.